4 Concrete
Washed sand plus washed stone, mixed with cement. Four jobs, one rule: use the biggest stone that fits.
- 4.1Casting a concrete floorA flat, load-bearing slab on the ground. 0–4 mm sand and 8–16 mm stone, on a 16–32 mm capillary break.
- 4.2Foundation concreteStrip and pad footings below the local frost line. 0–4 mm sand and 16–32 mm stone, or 8–16 mm with rebar.
- 4.3Setting posts in concreteFence, gate and pergola posts at least 0.9 m deep. 0–4 mm sand and 8–16 mm stone, on an 8–16 mm gravel base.
- 4.4Casting in formworkWalls, steps and plinths in formwork. Stone sized to the narrowest gap, with a checker.
The mixes in this chapter by volume, per m³ of concrete
| Mix | Used for | Cement | Sand | Stone |
|---|---|---|---|---|
| 1 : 2 : 4 | Floors, posts, formwork (4.1, 4.3, 4.4) | 285 kg | 0.62 t | 1.36 t |
| 1 : 3 : 5 | Footings (4.2) | 220 kg | 0.71 t | 1.31 t |
From the FAO Farm Structures concrete table (5.7 and 4.4 bags of 50 kg, naturally moist sand and stone). Strength depends heavily on the water: use only as much as you need to work the mix.
Why the stone size matters
Bigger stone needs less mixing water (Penn State), and less water gives stronger concrete that shrinks less, but the stone must fit. The largest stone may be no more than ¼ of the thinnest part (FAO), ⅕ of the narrowest form width and ¾ of the gap between bars (ACI 318), and the bar gap must be at least the stone plus 5 mm (EN 1992-1-1).
Chapter 4
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